Forgive me if I'm wrong, but it's not a minor difference: Linux is a kernel, Ubuntu is a distribution.
Linux - 1. a free Unix-type operating system. 2. (technical) the kernel of that operating system. See also GNU+Linux, GNU/Linux, Stallman R., bikeshedding, pedantry
END-PACKET
Ubuntu is a distribution of the Linux OS, so there is likely to be a large overlap in the questions - e.g. "How do I do X in bash (BTW, I'm on Ubuntu Randy Racoon)?" Many users may not want to ask a question outside the home forum of the distro they installed; conversly, most questions will apply to more than one distro. Merging these makes sense, except for community effects.
My point was that the two communities' lack of support for the merge is not really an application of "the narcissism of minor differences": the overlap exists, but it is not that important, at the moment. Perhaps in time, such a merge will be justified and users of those two sites might acknowledge that.
I hope this clarifies my first post for you.
As a topic of questions? I think not. I believe that most people when thinking "site for questions about Linux" would include both kernel and Ubuntu questions within the scope.
And of course a lot more - gnu tools, "which distro?" questions, advocacy, apps-related questions, libs, configuring the X server...